1. Field of the Invention
The present invention is applicable to a field in which it is intended to derive high-purity nickel sulfate from a nickel-containing acid solution, the high-purity nickel sulfate being usable as a battery material containing less amount of impurities, particularly magnesium and calcium.
2. Description of the Related Art
Nickel has been widely used as a material for stainless steel and corrosion-resistant alloys, and besides, in recent years, nickel has been frequently used as a material for nickel-metal hydride batteries and lithium-ion batteries used for hybrid electric vehicles, cell phones, personal computers, and the like.
Nickel used as these materials has been produced by mining mineral ores that exist in the form of sulfide ore or oxide ore, and smelting the mineral ores.
One example of methods for processing a sulfide ore is such that a sulfide ore is put into a furnace to melt; impurities are separated therefrom as slag to form a matte containing concentrated nickel; said matte is dissolved with sulfuric acid or hydrochloric acid; and impurities are separated from the dissolved solution to obtain a nickel solution. Then, the obtained nickel solution is neutralized, crystallized, or the like, whereby nickel salts, such as nickel sulfate and nickel oxide, are produced. Alternatively, sometimes nickel metals are produced by electrolytic winning or the like.
On the other hand, one example of methods for processing an oxide ore is such that an oxide ore is heated and melted together with a reducing agent, such as coke, to be separated from slag, whereby ferronickel, which is an alloy of nickel and iron, is obtained to be used as a material for stainless.
However, any of these smelting methods requires a large amount of energy and a great deal of time, effort and cost for separation of impurities.
Particularly, in recent years, high-quality ores are being depleted, and accordingly it has been hard to secure such high-quality ores, as a result, the nickel grade in ores available tends to be lower, and thus more time, effort and more costs have been required to obtain nickel from such low-grade raw materials.
Therefore, there has been recently developed a method of obtaining a nickel salt or nickel metal by subjecting low level oxide ores that have not been conventionally used as raw materials, to acid leaching under high temperature and pressure, and neutralizing the leached solution with an alkali such as slaked lime.
This method enables a low-grade resource to be used effectively and with relatively less energy, but, in the case where the above-mentioned nickel salts need to be obtained, there has arisen new problems that never have been found in conventional smelting methods.
For example, according to the above-mentioned smelting method using a furnace, most of magnesium contained in ores are distributed to the slag, and accordingly, less magnesium is distributed to the matte. As a result, the amount of incorporation thereof into nickel salts was limited to a very low level, and the incorporation hardly caused any problem.
On the contrary, according to the smelting method using high temperature and pressure leaching, magnesium and manganese are leached well with acid, and as a result, incorporation of the elements into nickel salts is also increased. Furthermore, in high temperature and pressure leaching, there is performed a neutralization operation, wherein a neutralizing agent is added to an obtained leached slurry to adjust the pH thereof and whereby components other than a target metal are precipitated and separated therefrom, and, in the case of using calcium hydroxide or the like, which is industrially inexpensive, as a neutralizing agent, an effect of the incorporation of calcium obtained after the reaction into nickel salts cannot be disregarded.
Particularly, in the case where nickel is used in a material for lithium-ion batteries or nickel hydride batteries, coexistence of nickel with magnesium, calcium, chloride ions greatly affects the characteristics of a battery finished as a product, and therefore, there is desirable a high-purity nickel salt which is obtained by avoiding the incorporation as much as possible from a stage of producing the nickel salt.
In the meanwhile, in order to obtain nickel sulfate, which is one of nickel salts, with high purity, another method may be considered, wherein, for example, nickel is once obtained as a metal by electrolytic winning or the like, and this metal is dissolved again in sulfuric acid, and then, the dissolved solution is concentrated to crystallize nickel sulfate. However, obtaining such metal requires a considerable amount of electric power and a correspondingly large scale of equipment, and therefore, in consideration of energy efficiency and costs, the method is not advantageous.
Furthermore, in many cases, a nickel-containing ore also contains cobalt. Cobalt is also a valuable metal and does not need to coexist with nickel, and therefore, cobalt and nickel are separated and individually recovered.
As an efficient and practical method of separating nickel and cobalt contained in a sulfuric acid solution, solvent extraction has been often employed. For example, Japanese Patent Application Laid-Open No. H10-310437 discloses an example of separating nickel and cobalt by extracting cobalt by solvent extraction using trade name: PC88A (manufactured by Daihachi Chemical Industry Co., Ltd.) as an extractant.
In the case of using PC88A as the extractant, the extraction behavior of magnesium and calcium resembles the extraction behavior of nickel, and therefore, when solvent extraction is applied to a solution containing high-concentration nickel, there occurs a problem of a decrease in efficiency of separating magnesium and calcium, such as a decrease in the extraction rate of magnesium and calcium.
On the other hand, Japanese Patent Application Laid-Open No. H10-30135 discloses a method for producing an aqueous solution of high-purity nickel which does neither include any sodium nor ammonia, from an aqueous nickel solution containing calcium, magnesium, cobalt and the like as impurities, by extracting and separating impurities in the aqueous nickel solution using an alkylphosphonic acid ester or an alkylphosphinic acid, each containing nickel, as an extractant.
According to the method proposed in Japanese Patent Application Laid-Open No. H10-30135, the method is such that nickel is extracted into an organic solvent having a high pH in advance, and the organic solvent into which nickel is extracted is made to come into contact with a nickel solution containing impurities, and there occurs an exchange reaction in which elements more easily extracted than nickel are transferred to an organic phase, while nickel in the organic solvent is transferred to an aqueous phase, whereby the impurities in the nickel solution can be removed.
Furthermore, the method is effective to prevent a product from being contaminated due to the incorporation of an impurity element, such as Na, contained in a pH adjusting agent into the nickel solution.
However, also in the solution purification step for nickel sulfate proposed in Japanese Patent Application Laid-Open No. H10-30135, magnesium in the solution resembles nickel in behavior, and accordingly it has been difficult to remove magnesium to the extent that nickel sulfate obtained may be used as a battery material.
Furthermore, in the case where a large amount of impurities, such as iron and aluminum, are contained in a nickel-containing material that serves as a raw material, a large amount of a neutralizing agent is required to separate the impurities therefrom using a process, such as neutralization, and in addition, there is a possibility that, when impurities are precipitated, valuables, such as nickel and cobalt, are also co-precipitated, thereby causing a loss, and hence it has not been easy to perform an efficient operation.
For these reasons, there has been desired a practical method capable of efficiently obtaining a high-purity nickel sulfate which has low-grade magnesium or chloride and is usable as a battery material, from a sulfuric acid solution containing a large amount of metal ions, such as magnesium, and chloride ions.
In such circumstances, an object of the present invention is to provide a removal method of an impurity-element for selectively removing magnesium from a nickel-containing acid solution and a method of producing high-purity nickel sulfate using said removal method.